Known is a method of resistance butt welding of items having large cross-sections (A.C. USSR 105251 B.I. No. 1, 1957, MPK V 23 K 11/2), where circumferential projections are made on parts being welded for concentration of the heat generated.
Known is a method of resistance butt and spot welding (Patent of Japan 45-22604), in which an insert in the form of a lens is placed between parts of different thickness to provide the possibility of their welding. This gave an opportunity to increase the contact resistance of the butt and to increase the heat localization.
The disadvantage of these methods is a short time of the contact resistance increase due to overheating and plastic deformation of an intermediate insert.
Known also is a method of resistance butt welding (A.C. USSR 1232422 B.I. No. 19 1986 MPK V 23 K 11/02), in which a machine lubricating oil is used as a protection of the welding zone and prevention of carbon burn-out in the butt.
Known too is a method of resistance welding using a flux (U.S. Pat. No. 1,437,764), in which the welding is performed under a flux layer, shielding the welding zone from the atmosphere and cleaning the surface of oxide films.
The main disadvantage of the above-mentioned methods is the fact that the edges of the parts are in a brought-together state before the pouring of a flux mixture or organic elements. Therefore the metal, during the welding process, is shielded only on the external surface of the joint, while gases and oxides, available at the edges of parts being welded, remain mainly in the butt. The external shielding has little influence on the quality of welding inside the butt due to the impossibility of flux and metallic powder to enter the butt of strongly clamped parts.
Also known is a method of resistance butt welding (A.C. USSR 1738539 B.I. No. 21 1992 MPK B 23 K 11/02), selected as a prototype, in which a current-conducting insert in the form of a powder of a self-fluxing current-conductive alloy or a mixture of a metallic powder with alloying and fluxing components is placed between the edges of parts before welding. The powder is maintained by a non-conducting bushing with a slot. During welding, the powder is forced out together with oxides and a molten layer through the bushing slot. This method guarantees the oxides removal inside the butt and increases the heat localization at the expense of a contact resistance of the metallic powder.
The disadvantages of this prototype are the non-uniformity of a thermal field in the joint cross-section due to a random nature of formation of contact spots in the metallic powder layer that leads to overheating and splashes of locally-overheated volumes of metal. Thus, the bushing application complicates the process of welding and does not allow the process to be automated.